1. Field of the Invention
This invention relates to diesel engine exhaust gas treatment and more particularly to the filtering of particulates from diesel engine exhaust gases using a catalyzed filter.
2. Background and Related Art
The question of how best to reduce the levels of particulate matter expelled to the atmosphere in the exhaust gases of diesel engines is currently of considerable interest. In this connection, it is desired to develop efficient and practical devices for removing substantial portions of particulates from the exhaust gases in diesel engine exhaust systems before permitting the exhaust gases to escape to the atmosphere.
It is known in the art to provide diesel engines with an exhaust filter which traps particulates from the exhaust gas stream during engine operation. The filters are generally made of porous, solid materials having a plurality of pores extending therethrough and having small cross-sectional size, such that the filter is permeable to the exhaust gases which flow through the filters and are capable of restraining most or all of the particulates from passing through the filter with the gas. The restrained particulates consist generally of carbonaceous particulates in the form of soot particles and reference herein and in the claims to "particulate" and "particulates" means such diesel engine-generated particles. As the mass of collected particulates increases, the flow rate of the exhaust gas through the filter is usually impeded, whereby an increased back pressure is encountered within the filter and reduced engine efficiency results. At this point, the filter is either discarded as a disposable/replaceable element or removed and regenerated by burning the collected particles off at temperatures in excess of 510.degree. C. so that the filter can be reused.
There is a desire in the art to more simply regenerate the particulate filter by continuous burn-off or incineration of the soot particles as they are trapped in the filter. However, experience has shown that in normal diesel engine operation, the temperature in the exhaust system varies substantially under different conditions of engine load and speed and that the temperatures in the filter hardly ever reach the 510.degree. C. temperature level required to incinerate the trapped particulate.
The art, as exemplified by U.S. Pat. No. 4,319,896, has attempted to rectify this situation by the use of a filter which embodies a catalytic material which raises the temperature of the hot exhaust gas stream. The art is not specific as to the catalytic materials, but teaches that, even at the raised exhaust temperatures effected by the use of these unspecified catalytic materials, these temperatures are still too low to effectively ignite the carbon particles retained in the filter. To effect burn-off of the particles in the filter, the filter must be periodically heated by electrical means to a temperature sufficiently high to burn off the retained soot. Even the use of an electrical burn-off device involves risk, for as noted in U.S. Pat. No. 4,319,896, care must be exercised during burn-off of the soot particles as the possibility exists that there will be too rapid a burning of the particulate and the filter may be heated to an excessively high temperature which can adversely damage the filter.
Federal Republic of Germany published Patent Application DE 3141713 (May, 1983) indicates that the use of a particulate filter which embodies a silver vanadate catalyst on a transition metal carrier such a aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide and oxides of rare earth metals, lowers the temperature at which ignition of the trapped particulate is initiated.